1. Field of the Invention
The present invention relates to a measuring method and apparatus using attenuated total reflection, such as, for example, a surface plasmon resonance measuring apparatus in which a particular substance in a sample is analyzed by attenuated total reflection induced by the excitation of surface plasmons. More specifically, the present invention relates to a measuring method and apparatus that uses attenuated total reflection, in which the state of attenuated total reflection at an area of measurement having a two-dimensional area is detected by a collimated light beam of a certain wavelength selected from a plurality of wavelengths.
2. Description of the Related Art
In a metal, free electrons oscillate as a group and a compressional wave called a plasma wave is generated. The quantized compressional waves generated on the surface of a metal are known as surface plasmons.
Various surface plasmon resonance measuring apparatus have been proposed for analyzing the properties of a sample by applying the phenomenon that the surface plasmons are excited by a light wave. Among these measuring apparatus, the one that uses a system called Kretschmann configuration is particularly well-known as described, for example, in Japanese Unexamined Patent Publication No. 6(1994)-167443.
Basically, the surface plasmon resonance measuring apparatus that uses the system described above comprises, for example, a dielectric block shaped like a prism; a metal film formed on one of the surfaces of the dielectric block and brought into contact with a sample; a light source for generating a light beam; an optical system for entering the light beam into the dielectric block at various incident angles including an angle that satisfies the conditions of total reflection and surface plasmon resonance at the interface between the dielectric block and the metal film; and a optical detecting means for detecting the state of surface plasmon resonance by measuring the intensity of the light beam totally reflected at the interface.
When a light beam enters the metal film of a surface plasmon resonance measuring apparatus configured in the aforementioned manner at a certain incident angle θsp which is not smaller than the total reflection angle, an evanescent wave having a distributed electric field is generated in the sample in contact with the metal film, and thereby surface plasmons are excited at the interface between the metal film and the sample. When the wave number matching is achieved, in which the wave-number vector of the evanescent light matches the wave-number vector of the surface plasmons, the evanescent light and the surface plasmons go into the state of resonance, and the intensity of the light totally reflected at the interface between the dielectric block and the metal film drops sharply, because the light energy is transferred to the surface plasmons. This drop in the intensity of light is generally detected as a dark line by the optical detecting means described above. Thus, the wave number of the surface plasmon is determined by the attenuated total reflection angle θsp, which is the incident angle that causes the attenuated total reflection.
In such a type of surface plasmon resonance measuring apparatus described above, the resonant wave number is measured by sweeping a range of incident angles of a single wavelength, or by entering a light beam at various incident angles. On the other hand, it may also be measured by sweeping a range of wavelengths with a fixed incident angle as described, for example, in “Surface plasmon resonance sensors based on diffraction grating and prism couplers: sensitivity comparison”; Sensors and Actuators B 54 (1999), P16 to 24.
As for the surface plasmon resonance measuring apparatus that employs the swept wavelength method, an apparatus comprising a light source for generating a light beam having a plurality of wavelengths; a wavelength selecting section for sweeping a range of wavelengths by sequentially selecting a desired light beam of a single wavelength from the light beam, an optical system for entering the selected light beam of the single wavelength into the dielectric block at an angle that satisfies the conditions of total reflection at the interface between the dielectric block and the metal film, and a optical detecting means for measuring the intensity of the light beam totally reflected at the interface is known as described, for example, in “Porous Gold in Surface Plasmon Resonance Measurement”; EUROSENSORS X111, 1999, P585 to 588.
The measuring apparatus described above detects the wave number of the surface plasmon by repeating the measurement through repeated wavelength sweeping with a fixed incident angle, and by detecting the attenuated total reflection wavelength λSP which is the wavelength that causes the attenuated total reflection. The resonance described above occurs only when the incident light beam is in a p-polarized mode. Accordingly, arrangements need to be made in advance so that the light beam enters in the p-polarized mode.
When the resonant wave number, that is, the wave number of the surface plasmon is detected, the dielectric constant of the sample maybe obtained. More specifically, the following relationship may be obtained, assuming that Ksp is the wave number of surface plasmons, ω the angular frequency of the surface plasmon, c the speed of light in vacuum, ∈m the dielectric constant of the metal, and ∈s the dielectric constant of the sample.
      Ksp    ⁡          (      ω      )        =            ω      c        ⁢                                                      ɛ              m                        ⁡                          (              ω              )                                ⁢                      ɛ            s                                                              ɛ              m                        ⁡                          (              ω              )                                +                      ɛ            s                              
When the dielectric constant ∈s is determined, the refractive index of the sample may be obtained based on a predefined calibration curve and the like. That is, by determining the attenuated total reflection angle θsp, or the attenuated total reflection wavelength λSP that causes the attenuated total reflection described above, the refractive index of the sample or characteristics related to the refractive index of the sample may be obtained.
A leaky mode measuring apparatus using, for example, a dielectric cladding layer is also known as a similar sensor that uses attenuated total reflection as described, for example, in “BUNKOH KENKYU”; Vol. 47, No.1 (1998) P 21 to 23, and P26 to 27. Basically, the leaky mode measuring apparatus comprises, for example, a dielectric block shaped like a prism; a cladding layer formed on one of the surfaces of the dielectric block; an optical waveguide layer formed on the cladding layer and brought into contact with the sample; a light source for generating a light beam; an optical system for entering the light beam into the dielectric block at various angles to satisfy the conditions of total reflection at the interface between the dielectric block and the cladding layer, and to cause attenuated total reflection by the excitation of a waveguide mode in the optical waveguide layer; and a optical detecting means for detecting the state of excitation of the waveguide mode or attenuated total reflection by measuring the intensity of the light beam totally reflected at the interface.
When a light beam is incident on the cladding layer through the dielectric block of a leaky mode measuring apparatus configured in the aforementioned manner at a certain incident angle not smaller than the total reflection angle, a certain light component of the light beam having particular wave number and incident angle passes through the cladding layer, and propagates along the optical waveguide layer in a waveguide mode. When the waveguide mode is excited in this manner, attenuated total reflection occurs, in which the intensity of the light totally reflected at the interface described above drops sharply, because most of the incident light is contained in the optical waveguide layer. The wave number of the guided light is dependent on the refractive index of the sample placed on the optical waveguide layer, so that the refractive index of the sample and other characteristics related thereto may be analyzed by determining the attenuated total reflection angle θsp that causes the attenuated total reflection described above. The refractive index or the characteristics related to the refractive index of the sample may also be analyzed by detecting the attenuated total reflection wavelength λSP by sweeping a range of wavelengths with a fixed incident angle as in the surface plasmon resonance measuring apparatus described above.
In the analysis of the properties of a sample with the surface plasmon resonance measuring apparatus, or leaky mode measuring apparatus, it may often be required to measure the properties of a plurality of samples under the same measuring conditions, or to obtain information on two-dimensional properties of a sample, thus the application of these apparatuses to these fields has been contemplated. Taking the two-dimensional property measurement for a sample using the surface plasmon resonance measuring apparatus, as an example, when a collimated light beam having a predetermined wavelength enters into a region of the interface having a two-dimensional area at a predetermined incident angle, the light component totally reflected from the section of the region where the refractive index of the sample is such that the attenuated total reflection is induced by the predetermined incident angle and wavelength is detected as a dark spot. Thus, the characteristics of the sample related to the two-dimensional distribution of the refractive indices of the sample along the interface may be measured by using a collimated light beam having a sizable cross section, and containing a plurality of wavelengths; selecting a desired collimated light beam of a single wavelength therefrom; entering the selected light beam into the interface between the dielectric block and the metal film; and detecting the distribution of the optical intensities on the cross section of the collimated light beam totally reflected at the interface as described, for example, in “Development of a Two-Dimensional Evaluation Method for Thin Layers Using Surface Plasmon Resonance”; Chemistry Letters 2001, P1312 to 1313.
The above description may also be applied to the leaky mode measuring apparatus except that the attenuated total reflection is induced by the excitation of the waveguide mode in the optical waveguide layer, instead of by surface plasmon resonance, so that the two-dimensional properties of a sample described above may also be obtained in the similar manner by using the leaky mode measuring apparatus.
The measuring apparatus described in the document entitled “Development of a Two-Dimensional Evaluation Method for Thin Layers Using Surface Plasmon Resonance” described above enters a collimated light beam having a desired single wavelength selected from a collimated light beam having a plurality of wavelengths into the interface between the dielectric block and the metal film, and detects the distribution of the optical intensities on the cross section of the reflected light beam by a optical detecting means, so that a wavelength selection filter needs to be placed in the optical path in the vicinity of the optical detecting means for passing only the light components having wavelengths close to the wavelength of the collimated light beam reflected at the interface in order to block stray light, such as ambient light. Further, when sweeping a range of wavelengths of the collimated light beam, the range of the wavelengths passing through the wavelength selection filter must also be swept over. Consequently, the wavelength selection filter becomes complicated and large, contributing to larger size and increased cost of the measuring apparatus.